1. Field
The present embodiments generally relate to the subject of drive mechanisms for compressors. More particularly, the present embodiments relate to the use of multiple motor drive systems to drive a single compressor string.
2. Description of the Related Art
Compressors, such as those used in refrigeration applications, for example in the production of liquefied natural gas (LNG), are often driven by a gas turbine, an electric motor, or a combination of both.
Industrial gas turbines used in LNG production may have been originally designed for use in other industries, such as the electrical power industry. These turbines are typically designed with specific outputs. Two designs that are available include (i) designs for the 50 Hz market, which operate at 3000 rpm, and (ii) designs for the 60 Hz market, which operate at 3600 rpm. These requirements are precise and speed variation is severely constrained. Frequency variability is on the order of about ±5%.
The designs of such gas turbines can be very limiting in that any deviation from the operational designs of 3000 rpm for 50 Hz or 3600 rpm for 60 Hz power may result in significant problems for the electrical users. For example, if the turbine and associated compressor are operating at 3100 rpm, but the electrical grid frequency is 50 Hz (3000 rpm), the generated frequency would be 51.7 Hz. Generating electrical power at 51.7 Hz may cause significant problems for users connected to the electrical system. Typically, frequency tolerance for electrical systems is on the order of only ±0.5 Hz.
Such turbines are designed to be most efficient when they are operated at maximum capacity, which can also reduce emissions and fuel consumption. To increase or decrease energy production the fuel flow rate can be varied. Operation of the gas turbine at lower fuel rates will generally reduce its efficiency and increase its emissions.
Starting a power-generating gas turbine is a simple task, because a starter motor is only needed to spin the gas turbine and generator up to speed from its powered-down state. Once at operating speed, the gas turbine provides all needed power and the starter can be disengaged.
Starting a compressor-driven gas turbine is more difficult compared to starting a power-generating gas turbine. The reason for this is that the compression load must also be taken upon the starter motor until the gas turbine is brought up to speed. In the case of LNG production, it is the refrigerant that flows through the compressor throughout the starting process. To overcome this obstacle a larger starter motor is required in compressor-driving gas turbine applications.
Gas turbines operate at different output ratings depending on the ambient temperature. This should be taken into account when such turbines are deployed. For warmer climates the turbine must be sized to overcome the adverse effects a higher temperature has on its ability to power the associated compressor in the hotter months. During cooler months less power may be required, and the turbine may produce excess energy that may not be needed.
The use of a motor to drive compressors has advantages. The motor can vary frequency much more readily than a gas turbine is capable of, which is useful in increasing efficiency and reducing waste energy. A motor can have a smaller footprint in a plant, making it possible to design smaller plants and/or transportable plants, such as those built on an offshore vessel or structure. A motor can draw power from an existing power grid, from a generator, or from other sources of power. A motor can be controlled by an adjustable speed drive ASD, capable of controlling the speed of the motor. An adjustable speed drive can also be referred to as a variable frequency drive VFD, a variable speed drive VSD, variable speed drive system VSDS, adjustable frequency drive AFD, or other variations which are considered inclusive in the use of the term ASD herein.
Another arrangement to drive one or more compressor is that of a motor and gas turbine combined. This arrangement can still have the constraints and problems inherent in utilizing a gas turbine.
A multiple motor assembly may be desirable in certain applications in order to address some of the limitations the aforementioned systems have. Higher torque control can be maintained throughout the drive system. Less waste and waste energy would be produced than in a system comprising gas turbines. Multiple smaller motors linked together can provide the power of a larger motor. Smaller motors can be more industry proven and tested than larger motors. With a multiple electric motor assembly if one motor is not operable the other motors can still supply power and the facility, depending on the embodiment, can still operate at a reduced capacity.